How the string light calculator works
String lights are never hung in a straight line. They droop, and the droop uses more lights than the distance between the two hooks. This calculator allows for it. Enter the distance between your anchor points, how far the lights droop in the middle, and how many hang points you have, and it gives you the length of lights you need, the best strands to buy, the number of bulbs, and the number of hooks.
Choose Straight run for one line of lights across a patio or yard, Zigzag over a patio for rows of lights that cover an area, or Around a perimeter for a deck, pergola or fence.
How much sag should string lights have?
A droop of about 3 to 6% of the span looks natural and keeps the strain on the hooks low. For a 20 ft span, that is 7 to 14 in. A tighter line looks harsh, and puts a lot of force on the hooks and the cord, which can pull a hook out of a wall or snap a cord. A deeper droop looks romantic, but uses more lights and hangs low in the middle.
| Span | Droop (about 5%) | Length of lights |
|---|---|---|
| 10 ft | 6 in | 10.1 ft |
| 20 ft | 12 in | 20.1 ft |
| 30 ft | 18 in | 30.2 ft |
| 50 ft | 30 in | 50.5 ft |
The extra length for a normal droop is small. The big difference comes when you hang the lights in several swags, because every swag adds its own droop and the hooks in between.
How high should string lights hang?
- At least 7 ft above anywhere people walk, measured at the lowest point of the droop.
- At least 14 ft over a driveway, where a car or truck may pass.
- For a patio or deck, 8 to 10 ft at the anchor points looks good, with the lowest point about 7 to 9 ft.
When do you need a support cable?
String light cords are not made to carry their own weight over a long distance. For a span of more than about 20 ft, hang a steel support cable between the anchors and attach the lights to it with clips or zip ties. The cable carries the weight, and the lights just hang. Use eye bolts or screw hooks that are rated for the load, and a turnbuckle to adjust the tension.
How many strands do you need?
String lights are sold in lengths of about 24 ft, 48 ft and 100 ft. The calculator finds the combination with the least waste and the fewest strands. Check the label to find out how many strands you can connect end to end. Many LED strands can join several together, but some only allow two or three.
Worked examples
A 20 ft straight run with a 1 ft droop
The lights need 20.1 ft, plus 3 ft for the lead to the outlet. That is about 24 ft, so one 24 ft strand is right, with about 17 bulbs at 18 in spacing.
A 16 ft × 12 ft patio with rows of lights 4 ft apart
That makes 4 rows across the 16 ft width, each with a 1 ft droop. Together with the links between the rows and the lead, you need about 80 ft of lights. One 100 ft strand covers it with 20 ft spare.
Common mistakes
- Buying a strand the same length as the span. The droop and the lead to the outlet use extra length.
- Hanging the lights too tight. A tight line puts too much strain on the hooks.
- Hanging them too low. The lowest point must be above head height.
- Not using a support cable on a long span. The cord will stretch and sag over time.
- Using indoor lights outside. Choose lights that are rated for outdoor use.
Tips
- Use a plug-in timer or a smart plug so the lights turn on at dusk.
- Choose LED lights, which use far less power and run cool.
- Plan where the plug will reach before you decide where the lights go. Use an outdoor-rated extension cord if you need one.
- Take a photo of the layout so that it is easy to put up again next year.
- Test the lights on the ground before you hang them.
